In a conventional head-up display apparatus of this kind shown in FIGS. 1 and 2, a projector 3 is disposed within a dash board 2 in front of a driver's seat in a vehicle.
The apparatus shown in FIG. 1 is called a simple reflection system, and the projector 3 is composed of only a display device 3a. An image of a display pattern on the display device 3a from the projector 3 is reflected by a front glass 4 acting as a combiner, and is then visualized by a driver 6 as a virtual image 5 in front of the front glass 4. With respect to the position of the virtual image 5, the distance a between the display device 3a and the front glass 4 as a combiner is equal to the distance a' between the front glass 4 and the virtual image 5.
The apparatus shown in FIG. 2 is called a distant display system, and the projector 3 is composed of a display device 3a and a lens 3b. An image of a display pattern on the display device 3a projected on the front glass 4 through the lens 3b is reflected by the front glass 4, and is visualized as a virtual image 5' in a predetermined position in front of the front glass 4. With respect to the position of the virtual image 5', the distance a between the display device 3a and the front glass 4 is less than the distance a" between the front glass 4 and the virtual image 5'.
In the conventional head-up display apparatuses mentioned above, the virtual images 5 and 5' are only formed in predetermined positions in front of the front glass 4. Therefore, the displayed image has no deep feeling so that, even when the image is distantly displayed, it is difficult to visually judge how far the displayed image is located since there is no compared object in the displayed image.
The head-up display apparatus for a vehicle was developed to avoid the danger of temporarily looking at a meter, etc., within the vehicle from the external field of view. The driver can visually overlap the external field of view and image data such as fuel, speed reflected by a front shield glass as a screen.
In the head-up display apparatus, a display device composed of a liquid crystal cell is used, and light is illuminated onto a display pattern formed in the display device, and is guided to the front shield glass through an optical system, and a virtual image is formed there.
FIG. 3 shows the basic construction of a display apparatus of this type. In FIG. 3, a liquid crystal cell 51 constitutes a display device, and a light source 52 illuminates the liquid crystal cell 51 from the back side thereof, and a lens 53 enlarges an image of the liquid crystal cell 51.
Namely, the liquid crystal cell 51 is disposed within the focal distance of the lens 53, and a display pattern formed within the liquid crystal cell 51 is illuminated by the light source 52, and a virtual image thereof is formed, through the lens 53, by a half mirror 55 disposed in front of a front shield glass 54, or by the front shield glass 54 as a half mirror.
Accordingly, the driver can visually overlap the virtual image and the external field of view as mentioned before.
When such a conventional display apparatus of this kind is used as a meter, it is necessary to display a strengthened image for the driver. The conventional meter of an instrument fitting type has various functions and designs for easily looking at the image.
When the virtual image display apparatus is constructed by the above viewpoint, the apparatus has the advantages in that the virtual image does not prevent the front field of view, but the display pattern formed within the liquid crystal cell is displayed on a plane in front of the driver, so that the displayed image does not have any sufficient three dimensional structure such as thickness, weight to be appealed as an image in comparison with the conventional apparatus, thereby giving no sufficiently impacted image to the driver.
Further, the size of the liquid crystal cell is limited to a considerably small one by the optical principle thereof, and the number of dots constituting picture elements is limited within the range of the limited size of the liquid crystal cell. Therefore, it is difficult to form a plurality of display patterns, and the displayed content and the pattern are rough in accordance with the size of the liquid crystal cell, thereby providing no sufficiently three-dimensional image.